Esterlein is correct. The lines appear in Act 4, Scene 3 of Macbeth, lines 246-7 in my edition of the play. Macduff has just been informed that Macbeth's soldiers have murdered his wife and all his children in revenge for Macduff's deserting him and going over to the forces being raised by Malcolm in England. It is Malcolm who speaks the words to Macduff. These lines have often been quoted over the centuries because they contain a truth that pertains not only to Macduff but to anybody who is suffering from grief.